The present invention is particularly directed to a polyphase alternator driven battery charging system for multiple voltage loads and particularly to such a system including a three-phase alternator connected to supply power at a first high direct current (D.C.) voltage to a load and at a second lower D.C. voltage to a different load.
In various engine driven applications such as over the road vehicles, off road vehicles, marine applications and the like, an internal combustion engine is the main power plant for driving of the corresponding vehicle. Various associated equipment for the internal combustion engine and the vehicle requires electrical power. The internal combustion engine will normally be provided with an electric starter. Electrically operated compressors, window heater defroster and other similar relatively heavy load devices may also be coupled to and used in the vehicle or other device. Uniformly, rechargable batteries are provided as the basic power supply. The battery provides power for starting of the vehicle as well as for operating the other loads in the absence of the vehicle operation. An alternator or generator is coupled to and driven by the engine for charging of the batteries to maintain stand-by power and to provide operating power to the loads during the operation of the vehicle and the loads. Certain of the electrical loads generally can be operated more efficiently at higher voltages. In heavy industrial vehicles, buses, large heavy trucks and the like, systems have been developed using 24 volt battery systems with a corresponding alternator for charging of the battery. Such systems are advantageously used for example for starting of the vehicle during cold weather conditions and the like. The 24 volt supply requires only half the current to produce the power output. Thus, the power loss in the circuit wiring and other connections, which is related to the square of the current, is significantly reduced. However, certain loads generally are operated more reliably and effectively from the conventional 12 volt supply. For example, vehicle lighting systems, sound systems and the like should be designed to operate at 12 volt. Although 24 volt lamps can be provided, the higher voltage lamps have fragile filaments and are often subject to breakage under rather normal operating conditions. As a result, practically all 24 volt systems used in vehicles provide a dual voltage supply to maintain 12 volts supplies for lighting and other accessories while providing a 24 volt supply for starting and operating of other selected loads. In present dual voltage systems; the battery unit includes the pair of 12 volt batteries with the 24 volt loads connected across the two batteries and the 12 volt loads connected across one of the two batteries. Switch means provide for selective connection of the loads to the battery unit. In operating the 24 volt loads, the two batteries are loaded and supply power to the load. The 12 volt loads draw power from only one of the batteries creating unbalanced loading and discharging of the two batteries in the system. The subsequent charging of the battery unit establishes a charging current through all the series cells thereof, or require use of separate rectifiers and switch connection to the alternator. The unbalanced loading and subsequent recharging is generally undesirable and various means have been suggested to provide controlled charging of the batteries using relatively complex and costly systems with the usual subsequent service and maintenance requirement. Even with the various available monitoring systems, there is a significant danger or possiblilty that one or more battery cells will fail as the result of excessive alternate loading and charging. The cost of the dual 24/12 volt system has also limited the application such that mass production of the 24 volt batteries is not competitive with the standard 12 volt for the standard automobiles, small trucks and the like. The 24/12 volt systems generally use complex electronic control systems to monitor and maintain appropriate charging of the batteries and connection of the loads to the respective circuits.
An alternator is presently widely used for the charging of the battery system in vehicles. The alternator is advantageously a three phase alternator in which the rate of rise of the current output decreases at increased speeds. However, in such alternators, a battery must be maintained connected to the alternator as a load to prevent the voltage from rising to a destructive level. Thus, as the load is reduced on the alternator, the voltage increases. The winding may be wound as a three phase wye winding.
The output of the alternator is rectified through a full wave rectifier to change the alternating current to direct current. A three-phase full-wave rectifier, of a well known construction, is shown consisting of paired uni-directional conducting devices, normally solid state diodes, connected in series. The diodes conduct in a single direction and by appropriate interconnection of the windings and the paired diodes, as shown, current flow is established from the three phase winding to a positive output line, through the battery unit and/or load units and a return line connected to the opposite side of the three-phase windings. Theoretically, the current flow in the winding should be so balanced and arranged that there is a zero potential and current available at the common center point of the Y-connected windings. Uniformly in practice, a voltage exists at the common point which has been used and connected into prior art electrical systems for operating of small auxiliary loads such as indicating lamps and the like. It has also been suggested that such common terminal can be connected into circuit through a separate pair of diodes to provide a D.C. output which can be connected into the battery and load circuit. The latter use may provide an improvement on the order of 15% in the efficiency of the system.
Typical prior art systems are shown in the following U.S. Pat. Nos. 3,962,621; 4,019,120; 4,156,171 and 4,153,869.